Interpretive Summary: Recently, an increasing number of producers are growing hybrid catfish resulting from mating of female channel catfish × male blue catfish (CB hybrid) instead of channel catfish. The information is very limited on the infectivity of Ichthyophthirius multifiliis (Ich), a severe fish parasite for blue catfish and CB hybrid. There is an urgent need for the information of susceptibility to this widely distributed parasite (Ich) for blue catfish and CB hybrid. In this study we compared the infection and fish mortality caused by Ich between channel catfish and blue catfish or between channel catfish and CB hybrid. The cohabitation challenge method was used in this study to evaluate fish susceptibility to the parasite between fish species, thereby decreasing the chance for variation between experimental units, such as the number of infective pathogens, exposure time, temperature, water quality, volume of flowing water, and amount of feed provided. The study results demonstrated that channel catfish, blue catfish and CB hybrid were all susceptible to Ich infestation. No difference was noted on parasite infection level and fish mortality caused by Ich infestation between channel catfish and blue catfish or between channel catfish and CB hybrid following exposure to different concentrations of infective parasites. The study results are important to aquaculture industry and can serve as a guide when selecting fish species to culture.

Technical Abstract:
Information on the infectivity of Ichthyophthirius multifiliis (Ich), a severe fish parasite that causes high mortality, is limited for blue catfish (Ictalurus furcatus) and catfish hybrids (CB hybrid) resulting from female channel catfish (Ictalurus punctatus) × male blue catfish crosses. The objective of this study was to compare channel catfish, blue catfish and CB hybrids on the infection level and fish mortality caused by Ich using a cohabitation model. To compare the susceptibility to Ich between channel and blue catfish, fish were exposed to 5,000 or 10,000 theronts per fish, respectively. There were no statistical differences in the infection levels and mortalities between channel catfish and blue catfish. Channel catfish showed an infection score of 2.9 and blue catfish of 2.7 when infected by theronts at 10,000 theronts per fish. The cumulative mortalities were 86.3% and 80.6 %, respectively for channel catfish and blue catfish when exposed to theronts at 5,000 theronts per fish. To compare the susceptibility to Ich between channel catfish and CB hybrid, fish were infected by cohabiting with 1 or 3 Ich-infected fish or exposure to Ich theronts at 2,500-10,000 theronts per fish. Channel catfish and CB hybrid showed similar infection levels of >150 trophonts/fish and infection duration of 7-8 days when cohabited with 1 or 3 Ich-infected fish. All channel catfish and CB hybrid exposed to theronts at the dose of 10,000 theronts/fish showed heavy infection of >150 trophonts/fish. The cumulative mortalities were 90% and 80% for channel catfish and CB hybrid after exposure to 2,500 theronts per fish. No statistical difference was found in the infection levels and mortalities using two infection methods between channel catfish and CB hybrid. Overall results indicated that CB hybrid were as susceptible to Ich as channel catfish or blue catfish.